1. Field of the Invention
The present invention relates to a transmission power control method and a transmission/reception apparatus, which are used in a CDMA (Code Division Multiple Access) that is a digital radio communication and so on, capable of controlling transmission power optimally for a battery saving in a mobile station apparatus and so on.
2. Description of the Related Art
In a conventional transmission/reception apparatus using a transmission power control, the transmission power is controlled to be increased in the case where a reception quality decreases by a decrease of a reception level due to fading, or the like, and to be decreased in the case of a good reception quality.
FIG. 1 is a block diagram illustrating a schematic configuration of a conventional CDMA transmission/reception apparatus. In FIG. 1, transmission data 1 is subjected to error correction coding in error correction coder 2, and interleaved in interleaver 3. Then the data is spread with a predetermined spreading code in spreader 4, demodulated in demodulator 5, amplified in transmission amplifier 6, and transmitted from transmission antenna 7.
At this time, transmission amplifier 6 amplifies the transmission power to the level set by transmission power setter 8. This example illustrates an example of diversity reception. Therefore, in a receiver, signals are received in first reception antenna 11 and in second reception antenna 12, detected in first detector 13 and in second detector 14 respectively, despread in first despreader 15 and in second despreader 16 respectively, then synthesized in synthesizer 17.
In the case where a plurality of paths present in a communication link due to the effect of delayed versions and so on, RAKE combining is also performed in synthesizer 17. The synthesized data are deinterleaved in deinterleaver 18, subjected to error correction in error correction decoder 19 to obtain reception data 20.
In the receiver, slot quality detector 21 detects a slot quality of reception data 20. In detail, at step ST1 of a flow chart illustrated in FIG. 2, the quality of the slot (a block of data with a certain length) is detected. At this step, as illustrated in FIG. 3, it is decided whether or not the slot quality (reception quality) 31 is higher than threshold value 32.
Based on the decision result, power controller corresponding to reception quality 22 (hereinafter abbreviated as power controller-CRQ 22) controls the transmission power to be decreased at step ST2 in the case where reception quality (reception level) 31 is higher (better) than threshold value 32. On the other hand, power controller-CRQ 22 controls the transmission power to be increased at step ST3 in the case where the reception quality is lower than the threshold value. In detail, power controller-CRQ 22 provides a control as illustrated by rectangle line 34 in FIG. 3. Then, at step ST4, transmits power control information is transmitted to transmission power setter 8 in a transmitter.
FIG. 3 is a diagram illustrating the control of power controller-CRQ 22 for reverse link signals (signals transmitted rom mobile station MS to base station BS). In FIG. 3, the fading between transmission side MS and reception side BS illustrated by curve 36 and the transmission power of mobile side MS illustrated by curve 38 are almost in inverse proportion.
Thus, it is possible to keep the reception quality almost constant, thereby allowing the improvement of the error correction ability in the case of using the error correction code having high resistance to random errors, such as convolutional code. However, there is a factor that a transmission power control error or interference volume changes, interleaving are performed to make errors further random.
According to the manner described above, the transmission power control corresponding to the reception quality always allows using of the minimum level of transmission power needed to keep the quality constant, thereby resulting in a battery saving of the mobile station. In addition, that also allows improving of system capacity because the interference volume is decreased by suppressing an unnecessary transmission power in the CDMA communication system.
However, in the conventional reception/transmission apparatus descried above, the transmission power is controlled to be increased in the case where the reception quality decreases by the decrease of the reception level due to the fading, or the like, and to be decreased in the case of the good communication quality. In the case where the reception level decreases due to the fading, it is necessary to increase the transmission level to tens of dB to transmit, which requires a transmission amplifier to have a large dynamic range. However, especially in the mobile station, the requirements for a battery life and specification on amplifiers have become severe, which makes it difficult to use an amplifier with the large dynamic range. In addition, the increase of the level makes the instant interference (to signals of other users) high in the CDMA communication.
The object of the present invention is to provide a transmission power control method and a transmission/reception apparatus capable of satisfying severe requirements for a battery life and specification on amplifiers, and of decreasing interference to signals of other users in a CDMA communication.
The present inventor was interested in the fact that an increase of transmission power did not improve an quality to an expected degree when a power level was decreased depending on level variations due to fading, and found the fact that an excess increase of the transmission power can be prevented by decreasing the transmission power to abandon such portion of communications in the case of level variations due to the fading, thus resulting in the present invention.
That is, the principle of the present invention is as follows. The transmission/reception apparatus in the reception side detects the reception level and provides the power control to the transmission side to increase the transmission power in the case where the detected reception level is higher than a threshold value, and decrease the transmission power in the case where the detected reception level is lower than the threshold value, and the transmission side thus performs a transmission by the power according to the power control.